USMLE

Homocysteine Metabolism

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Biochemical Pathways
  1. Glycolysis
  2. Cori Cycle
  3. De Novo Purine Synthesis
  4. De Novo Pyrimidine Synthesis
  5. Purine Salvage
  6. Purine Excretion
  7. Ethanol Metabolism
  8. Pyruvate Metabolism
  9. HMP Shunt (Pentose Phosphate Pathway)
  10. Galactose Metabolism
  11. Sorbitol (Polyol) Pathway
  12. Urea Cycle
  13. Alanine (Cahill) Cycle
  14. Catecholamine Synthesis & Breakdown
  15. Homocysteine Metabolism
  16. Fatty Acid Synthesis (Citrate Shuttle)
  17. Fatty Acid Breakdown (Carnitine Shuttle)
  18. Propionic Acid Pathway

Homocysteine can be metabolized down two different pathways: (1) methylation to methionine, (2) or transsulfuration to cystathionine with the eventual formation of cysteine.

Production of methionine from homocysteine is catalyzed by methionine synthase, which requires the Vitamin B9 (Folate)-derived 5-MTHF and Vitamin B12 (Cobalamin). Therefore, decreases in either folate or cobalamin can lead to increases in homocysteine (see Vitamins B9 and B12 Deficiencies). Notably, 5-MTHF is regenerated by MTHFR, and defects in MTHFR can lead to excess homocysteine and homocystinuria (see Homocystinuria - coming soon).


Production of cystathionine and eventually cysteine occurs through a different set of reactions. Namely, cystathionine synthase combines homocysteine and serine to produce cystathionine, using Vitamin B6 (Pyridoxine) as a cofactor. The cystathionine is then cleaved to produce cysteine.

Find this Homocysteine Metabolism mnemonic and more Biochemical Pathways mnemonics among Pixorize's visual mnemonics for the USMLE Step 1 and NBME Shelf Exams.